A drill head of this kind is composed of a roughly cylindrical head body 1 provided with cutting blades 3A to 3C along end walls 11a and 12a which extend in a common radial direction of chip inlet ports 11 and 12 which are opened in large and small fan shapes at a front end face thereof, a cylindrical screw shank 2 provided with a male thread 21 in an area which is slightly to a rear portion on an outer circumferential face thereof, as shown in FIGS. 5A and 5B, for example. The cutting blades 3A to 3C are generally formed by brazing cutting blade tips 30a to 30c made of cemented carbide to cutting blade mounting seats 13a to 13c concavely provided on the end walls 11a and 12a of the head body 1 made of steel. In FIGS. 5A and 5B, the head body 1 is composed of guide pads 4 made of cemented carbide and brazed to respective guide pad mounting seats 14 concavely provided at two locations at a front portion of the outer circumferential face thereof, and a pair of chucking groove portions 15 formed at radially opposed positions at a rear portion of the outer circumferential face thereof.
When using the drill head 10, as shown in FIGS. 6 and 7, a screw shank 2 is inserted into and coupled in threading relation with a distal end portion of a female thread 51 of a circular tubular tool shank (also referred to as a boring bar) 5 of a deep-hole cutting drill. At this time, outer circumferential parts at both front and rear sides sandwiching the male thread 21 portion of the screw shank 2 are adhered closely, as bounded circumferential face portions 22 and 23, to bounded inner circumferential portions 52 and 53 of the tool shank 5. As a result, the both components 10 and 5 are firmly coupled and integrated, while the insides of the head body 1 and the screw shank 2 and a hollow interior of the tool shank 5 are communicated, constituting a chip discharging passage 16. It is noted that the bounded circumferential face portion 22 at the front side is configured to be narrow and is greater in diameter than a non-bounded circumferential face portion 24 that is in front of it. On the other hand, the bounded circumferential face portion 23 at the rear side occupies the whole from the male thread 21 portion to a bordering portion at the rear end in the screw shank 2.
A deep-hole cutting is carried out by coupling the tool shank 5 with a spindle of a machine tool to be rotatingly driven or reversely by rotating a work material W. In this case, however, a coolant is supplied in the manner of an external supply. Therefore, as shown in FIG. 6, using a coolant supply jacket 6 that encloses the tool shank 5 oil-tight, a coolant C is introduced into the jacket 6 from a feed port 6a with high pressure, with the jacket 6 press-contacted with a work material W via a seal ring 61. The introduced coolant C in this manner is supplied to a distal end side of the drill head 10 through a gap T between the outer circumferential face of the tool shank 5 and the inner circumferential face of a cutting hole H. Together with a chip S generated in a cutting region, the coolant C is entered into a chip discharging passage 16 from the chip inlet ports 11 and 12 of the drill head 10 and then discharged outside from a proximal end side of the tool shank 5.
In the manufacturing of such a drill head 10, it was conventionally common that a head body 1 and a screw shank 2 were manufactured individually and the both were welded and integrated. More specifically, the head body 1 provided with cutting blade mounting seats 13a to 13c and pad mounting seats 14 and 14 as shown in FIGS. 8A and 8B was manufactured by casting, etc., while the screw shank 2 was manufactured by cutting and threading a cylindrical member. Cutting blade tips 30a to 30c and guide pads 4 and 4 are brazed to the mounting seats 13a to 13c, 14 and 14 of the head body 1, thereafter concentrically welding the front end of the screw shank 2 to the rear end of the head body 1, as shown in FIG. 9.
In the conventional manufacturing method, however, the screw shank 2 which has been finished into a perfect circle is easily deformed into an ellipse due to thermal strain involved in the welding of the head body 1 and the screw shank 2, and its deformation amount often reaches 100 μm. Accordingly, there was a problem that working accuracy by the obtained drill head was reduced. There was also a drawback that since the cutting blade tips 30a to 30c were brazed, positioning accuracy of a blade edge was difficult to obtain and reduction of cutting performance resulted especially in a case where the cutting blade 3A in the central section was off the shaft center of the head. Although it is not impossible to correct the blade edge accuracy by post-finishing cutting, not only is its operation excessively difficult but also manhours are increased, which results in a great rise in manufacturing costs.